Switch to shared DINOv3 WEB encoder (saves ~4-5 GB VRAM)

- Single DINOv3 WEB for both drone and satellite branches (shared_encoder=True default)
- One set of MONA adapters instead of two: 7M trainable vs 14M
- Total params: 438M (was 748M), trainable: 10.6M (was 17.6M)
- Asymmetric mode still available via shared_encoder=False
- Add gradient accumulation (grad_accum_steps, --grad-accum CLI flag)
- Update model summary in README

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

conf/gtauav_balanced.gin

@@ -25,6 +25,7 @@ TrainConfigGTAUAV.device = "cuda"
 # ---- Model ----
 TrainConfigGTAUAV.init_gate = 0.7
 TrainConfigGTAUAV.baseline_mode = False
+TrainConfigGTAUAV.shared_encoder = True
 
 # ---- Loss ----
 TrainConfigGTAUAV.tau_init = 0.07